In digital still cameras, digital video cameras, or electronic apparatuses with an imaging function (for example, smart phones and tablet terminals), an actuator for positioning a focusing lens is provided. Such an actuator adopts a stepping motor system, a piezo system, a voice coil motor (VCM) system or the like.
A VCM can generate a driving force according to the direction of a current flowing through a coil. A spring return system and a bidirectional drive system are known as VCM drive systems. A VCM with a spring return mechanism is structured to generate a driving force in a first direction by supplying a drive current to a coil and generate a driving force in a second direction, which is opposite to the first direction, by a force of a spring attached to a mover of the VCM. That is, electrical driving and mechanical driving are used in combination. When driving the VCM with the spring return mechanism, the drive current is supplied to the coil only in one direction, thereby simplifying a drive circuit. On the other hand, a drive circuit capable of sourcing and sinking a drive current from both ends of the VCM, like an H bridge circuit, is used in the bidirectional drive system. In the bidirectional drive system, a driving force in positive and negative directions can be obtained by switching the direction of a coil current.
The present inventor has studied a voice coil motor installed in an electronic apparatus and has found the following problems. FIGS. 1A to 1D are views showing a usage form of an electronic apparatus. The electronic apparatus 500 is, for example, a smart phone and has a camera (lens) 530 provided on a surface S1 opposite to a display in a housing. In FIG. 1A, the surface S1 is a vertical surface. In FIG. 1C, the surface S1 is a horizontal surface. FIGS. 1C and 1D show a state of a focusing lens 520. The focusing lens 520 is mechanically connected to a return spring 522. A voice coil motor (not shown) drives the focusing lens 520 along an arrow (stroke direction) 524.
As illustrated in FIG. 1C, in the state of FIG. 1A, gravity acts on the lens 520 in the direction perpendicular (indicated by an arrow g) to the stroke direction 524. Therefore, the influence of gravity on the driving of the voice coil motor may be neglected. On the other hand, as illustrated in FIG. 1D, in the state of FIG. 1B, gravity acts on the lens 520 in the same direction (indicated by an arrow g) as the stroke direction 524. That is, the gravity acts in a direction to displace the lens 520.
FIG. 2 is a block diagram illustrating a focusing system of the electronic apparatus 500. The system includes a lens module 502, an imaging element 504, an image processor 506, and a CPU (Central Processing Unit) 508.
The lens module 502 is provided to realize an autofocus function and includes a focusing lens 512 and an actuator 510. The lens 512 is movably supported in an optical axis direction. The actuator 510 controls the position of the lens 512 based on a command value S1 from the CPU 508.
Light (an image) which passed through the lens 512 is incident onto the imaging element 504. The image processor 506 reads image data from the imaging element 504. Based on the image data read by the image processor 506, the CPU 508 determines a target position of the focusing lens 512 so that the image passed through the focusing lens 512 is formed on the imaging element 504, and outputs the command value S1 corresponding to the target position to the actuator 510. The autofocus system may be a contrast system or a phase difference detection system.
In the system of FIG. 2, regardless of the posture of the electronic apparatus 500, in other words, without being affected by gravity, the same processing is performed. For example, in the contrast system, the command value S1 indicating the position of the lens 512 is feedback-controlled so that the contrast of the image captured via the focusing lens 512, which is affected or unaffected by gravity, is increased. A drive circuit (not shown) of the actuator 510 also supplies a drive current corresponding to the command value S1 to a VCM, regardless of the posture of the electronic apparatus 500.
Such control has a problem that, depending on the posture of the electronic apparatus 500, it takes a long time until the focusing lens 512 is stabilized at the optimum position by feedback. This means that a position command signal is repeatedly transmitted from the CPU 508 to the drive circuit of the actuator 510, which causes another problem of increased power consumption. Although the spring return system has been described here, the same problem may occur also in the bidirectional driving system. In addition to the autofocus, similar problems may arise in various actuators that are affected by the posture of the electronic apparatus.